1. Field of the Invention
The present invention relates to a multiple light source unit and an optical system using the same, and more particularly to a light source apparatus for converting light beams emitted from a plurality of light sources into a single light beam to be emitted from a specific position. For example, the multiple light source unit is used in a particle analyzing apparatus for radiating light onto a particle flowing in a fluid for detection of its scattered light or transmitted light and for capturing an image of the particle.
2. Description of the Related Art
A conventional multiple light source apparatus is known in which light beams emitted from a plurality of laser light sources are converted into light beams having a common optical axis by means of a half mirror (See, for example, Japanese Unexamined Patent Publication No. SHO 56(1981)-67756).
However, such a conventional light source apparatus lacks compactness and also requires a cumbersome adjustment. Moreover, loss of light due to the half mirror is inevitable. Further, if an image is captured using laser light, there are generated interference fringes, Fresnel diffraction, Fraunhofer diffraction, and the like due to coherence of the laser light with respect to space and time, thereby degrading an image quality.
On the other hand, the properties required in a light source are as follows:
(A) High uniformity of radiation intensity, PA1 (B) High electro-optical energy-conversion efficiency, PA1 (C) Small size and small weight, PA1 (D) Facility in handling emitted light, and PA1 (E) High output stability and less change with time.
However, these properties are contradictory to each other as shown below.
(A) High uniformity of radiation intensity
A high uniformity of radiation intensity in a field of view means that the coherence is sufficiently low. Therefore, the high uniformity in the radiation light intensity cannot be obtained unless a light beam of a specific angle emitted from a light source having a sufficiently low coherence is used.
(B) High electro-optical energy conversion efficiency
A light source having a high energy-conversion efficiency means a light source having a high coherence. This is contradictory to the property of (A). The reason is that uniformity in the radiation light intensity cannot be obtained unless a light source having a sufficiently low coherence, i.e. a light source having a low electro-optical conversion efficiency, is used.
Moreover, the fact that only a specific light beam from the light source can be used further decreases the conversion efficiency. The electro-optical conversion efficiency of a halogen lamp, a xenon lamp, or a flash lamp to be used as a microscope light source is typically several percent, and the amount of light actually used for capturing images is less than 1%. On the other hand, it is well known in the art that a coherent light source such as represented by a semiconductor laser has a conversion efficiency of 10 to 40%. However, if a light source having a high coherence is used as a light source for capturing images, it is not possible to obtain a clear image due to an influence of diffraction, interference, speckles, or the like.
(C) Small size and small weight
A small size means use of a coherence light source having a high electro-optical energy-conversion efficiency shown in (B). However, this is contradictory to the property of (A). Moreover, it is necessary to compactly arrange optical elements including light source elements and coherence reducing elements, without creating a dead space.
(D) Facility in handling emitted light
This means that the light source has a precise optical axis to be handled with high precision. Therefore, it is necessary that all the light beams from the light source are concentrated within a specific angle range.
(E) High output stability and less change with time
High output stability and less change with time mean adoption of a small and thermally stable structure having a high electro-optical conversion efficiency.
Therefore, light sources including a laser and an SLD can satisfy the items (B), (C), and (D), but not the item (A); whereas incandescent light sources can satisfy the property (A) under limited conditions, but not the other items.